This research aims to investigate the thoracic chemoreceptors (aortic bodies) of adult anesthetized dogs. I am particularly interested in those group 4 bodies which are supplied entirely or partly with blood by an artery or arteries which arise from the left coronary artery or one of its major branches. These bodies lie between the aorta and pulmonary artery. I will devote my major effort in a detailed study of the mechanism involved in a reflex depression of arterial pressure which follows elevation of pressure in the small coronary vessel which supplies these thoracic receptors. I am particularly anxious to determine whether this depressor reflex is due to an inhibition of chemoreceptor function as a result of increasing the chemoreceptor blood supply or whether it is due to a stimulation of baroreceptors which may be present in the vessels supplying the chemoreceptors, or both. I will attempt to evaluate these two possibilities by a detailed study of the alterations in both baroreceptor and chemoreceptor impulses recorded from the aortic nerves following changes in perfusion pressure and alterations in the PO2, PCO2 and pH of the blood which perfuses these receptors. I also hope to study possible reflex coronary resistance changes induced by stimulation of these chemoreceptors and possibly of the long term effects of permanent occlusion of their coronary blood supply.